1. Field of the Invention
The present invention relates to optical systems for image sampling and, in particular, to an optical system and method for sampling a secondary, optical image in a video projection system for the purpose of controlling the projected primary image.
2. Discussion of the Prior Art
To project a high quality, high resolution display utilizing a color video projection system, it is necessary to converge accurately and to maintain in registration the individual projected primary color images over the entire display area.
A color projection display system which provides for image registration is disclosed by Ledebuhr, "Full-Color Single-Projection-Lens Liquid-Crystal Light-Valve Projector", SID 86 Digest, p. 381.
In the projection display system described by Ledebuhr, illumination light from a xenon arc lamp is directed toward a liquid crystal light valve system through a reflector and illumination optical system. The illumination light passes through a first beam-splitting polarizing prism that prepolarizes the light. S-polarized light is rejected from the system at this point and p-polarized light is transmitted. A second identical beam-splitting polarizing prism, oriented at 90.degree. with respect to the first prism, sees the light incident from the first prism as s-polarized and, therefore, reflects it toward the liquid crystal light valves. The s-polarized light from the second prism is first directed through a 45.degree. blue reflecting dichroic filter. Green and red wavelengths are transmitted by this filter and next encounter a 45.degree. red reflecting dichroic filter. This second filter reflects the red light and transmits the green. The green light is then transmitted directly to a "green" liquid crystal light valve. The incoming red and blue light beams are each reflected to their respective liquid crystal light valves. Thus, the s-polarized illumination light from the second prism is split into three individual primary color beams (blue, red, green), each of which enters a separate reflective liquid crystal light valve for its corresponding primary color channel. The light from an image on an individual CRT associated with each color channel is directed to the photoconductive region of its light valve. Thus, upon reflection by the liquid crystal light valves, the outgoing light contains a polarization modulated image that is a replica of the image on the CRTs. This light then retraces its way back through the color filter assembly. The red and green modulated beams are recombined at the red reflecting filter and these are then recombined with the blue modulated beam at the blue reflecting filter. The light from the three channels now shares the same common axis that it did before polarization modulation. This recombined light passes back through the second polarizing prism where p-polarized light is transmitted and s-polarized light is reflected back toward the arc lamp illumination light source. The light transmitted by the second prism now consists of modulated light containing the primary color light image which is passed through a wide-angle projection lens to a display screen.
As stated above, to provide a high quality, high resolution display image, it is necessary to maintain accurately in registration the three individual projected primary color images over the entire screen area. The CRTs of the system described by Ledebuhr contained focus and astigmatism coils for spot shape programming, as well as additional deflection coils which are used to dynamically shape the three images. While this produces a full-color registered image on the display area, thermal and electrical drifts can cause image misregistration.
In order to reduce the drift to zero, the system described by Ledebuhr includes a feedback loop which is used to maintain the registration of the three primary color images. Three position-sensitive photodetectors are mounted at the display screen, outside the display area, to monitor the position of three projected target patterns that are generated during the vertical blanking interval of the display. Position signals from these three sensors are used to provide correction signals to the CRT deflection coils, which independently position the location of each of the three color images.
A major deficiency of the image registration correction technique described by Ledebuhr is the fact that it greatly reduces the mobility of the projection system. That is, since the three photodetectors utilized for image correction are mounted at the projection screen, which in most cases is permanently mounted, the projection system can be utilized only in conjunction with that screen or one that includes similar sensors. It would be highly desirable to provide a projection system which is transportable and still provides image registration correction.